Limiting Reactant

Limiting Reactant
(aka limiting reagent)
Excess Reactant
Theoretical, Actual, and
Percentage Yield
Limiting Reactant
(aka limiting reagent)
Limiting reactant: The substance you run out
of first, that is consumed first--The reactant
that limits the amount of other reactant(s)
that can combine and the amount of
products that can form
Excess Reactant
EXCESS REACTANT:
A substance that is not used up completely in a
reaction. It’s what is left over after you run out
of the limiting reactant. It is present in excess,
more than enough.
Theoretical, Actual, and
Percentage Yield
Theoretical yield: the calculated maximum amount of
product from a given amount of reactant. In theory,
the reaction should yield this amount.
Actual yield: the measured amount of product actually
obtained from a reaction
Percentage yield: the ratio of actual yield to
theoretical yield x 100% (like any percent)
S’mores:
20 chocolate chips
30 marshmallows
40 cookies
You can make 10 s’mores based on the chocolate chips
You can make 15 s’mores based on the marshmallows.
You can make 40 s’mores based on the cookies.
How many s’ mores can you make? 10 YIELD
What do you run out first? Chocolate chips
LIMITING REACTANT
How much of the other substance are left
over? If you make 10 s’mores, you use 20
marshmalllows and have 10 left. You use 10
cookies and have 30 left. EXCESS REACTANT
S’mores:
How many s’ mores can you make? 10 YIELD
What do you run out first? Chocolate chips
LIMITING REACTANT
How much of the other substance are left
over? If you make 10 s’mores, you use 20
marshmalllows and have 10 left. You use 10
cookies and have 30 left. EXCESS REACTANT
If you get sneaky and eat a chocolate chip and
can only make 9 s’mores, you have not made
the EXPECTED or THEORETICAL YIELD. The
ACTUAL YIELD is 9/10 x 100% or 90%
Limiting reactant
• Calculate in several ways
– Choose a product and calculate the quantity of
that substance that would be produced by each of
the starting quantities of reactants.
• Depending on the problem, you may calculate for
moles (the easiest and shortest calculation), grams,
particles, or volume.
• The starting substance that yields the lowest quantity is
the limiting reactant.
• EXAMPLE
Limiting reactant
• If no theoretical yield is required, you may use
this shortcut. You are never required to use it.
– Convert starting unit to moles
– Use the coefficient for each starting substance to
determine how many “reactions” you could get from
each substance.
– The starting substance that yields the fewest
“reactions” is the limiting reactant.
– This is similar to figuring out how many batches of a
recipe you can make from the ingredients on hand.
– EXAMPLE
Excess Reactant
• Set up a stoichiometric calculation from your product
(yield) or limiting reactant to the excess reactant to
figure out how much of the excess reactant is actually
consumed in the reaction. Calculate for the units asked
for in the problem.
• Subtract the amount of the excess substance actually
consumed from the amount of the substance added.
This is the excess reactant quantity.
• Amount added-amount consumed = excess
• Choose units based on your problem (moles, gram,
particles, volume)
EXAMPLE
Theoretical yield, actual yield, and
percentage yield
• Real world reactions do not always go to
completion to produce the expected or
theoretical yield.
• The actual yield is given to you in the problem
or determined experimentally.
• Actual yield/ theoretical (expected) yield x
100% = Percentage yield
• EXAMPLE